I've never had an issue with lag or frame rate dropping when playing this game. I usually play the game in cycles of a few months and take a few months off. However, I am noticing on almost every official server I play on the game will start to skip around and appear to be lagging during high-number enemy situations like certain level events. Like I've said before I've never had this problem with the game before or really any other game. I asked fellow players if they were experiencing skipping and appearances of "lag" and they said they were. Our pings were all in the green (mine around 24). Is there a server problem I am not aware of? Is there a remedy?

Official servers suck, but what you're talking about doesn't sound like framerate issues, more like stuttering. If you're playing on best available dedis, they might be running mods that force the game to do things it's not meant to do (e.g. spawning way too many infected at once) which might result in performance issues regardless of how powerful your hardware is (engine limitations). There's no reason as to why you should be experiencing FPS issues all of a sudden during normal gameplay if you never did before.

Green ping doesn't always mean a stable server. People have mentioned that Pinion servers (what you usually get now when selecting Official Dedicated) are dumbed down to keep CPU usage low, and refresh less often between clients and the server (tickrate) which all cause more lag and stuttering than the old Valve Official Dedicated servers did.